590 Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 (5-500 um), as shown in Fig. 6b. (3) Type C the whisker growth on the upper side of the nickel Whiskers from location III have similar size as substrate cylinder. However, in a vertical reactor shown in Fig. 6c. The difference of whisker mor- blows vertically upwards, a spout form resull o s Type B, but they grew as a dot-distribution, as along with the greatly reduced gas inlet, a fluid phology at different locations may be due to the produce a fast fluid along the annulus where the difference of contacting time and axial effective formation reaction of TiC whiskers occurs, the re linder.According to the ng the nickel substrate cy- action gas velocity in the vertical tube increases vapor concentration al gas movement with jet inlet This not only eliminates the radial laminar, and gas [11-13], which is similar to the movement of makes the radial concentration distribution of va vapor phase in the present experiment, from the por phase more homogeneous, but also accelerates bottom of the annulus in the vertical reaction tube the CVd process of mass transition limit On the to its top, the velocity of the vapor phase increases, other hand, apart from the fast gas blowing verti and the residence time of the vapor phase decreases. cally upwards, part of the vapor phases spouting Therefore, the effective vapor concentration and from the gas inlet diverges into the annulus. Com- the time of flowing vapor phase contacting the pared to the vapors directly flowing through the growing whisker fronts decrease from the inlet to substrate in the horizontal reaction tube, a higher the top. In location L, the effective vapor concentra- percentage of the vapor phase is taken to form the tion and the contacting time is the biggest, so the deposits in the vertical reaction tube due to the whiskers grow thick and long In location Il, the divergence of the vapor jet. This increases the col vapor concentration of the reactant is enough to lision of the vapor species on to the growing fronts form Ni-Ti binary eutectic liquid droplets as nu- and accelerates the mass transition of the CVD cleaton sites of TiC whiskers, while the flowing process. Therefore, the yield of the TiC whisker vapor phase cannot completely contact the whisker prepared by the modified CVD method increases tips. Thus in this location, the whiskers are thin and significantly short(Fig. 6b). In location Ill, the contacting time and the effective concentration of vapor phase are the least, the whiskers grow as a thin, short, dot- 4. Conclusions distribution due to incomplete growth and few nu- cleaton sites Quality TiC whiskers with a high yield are pre- From Table 1, it shows that the whisker yield in pared by a modified CVD method. The major fac the modified CVD method increases several times tors affecting the whisker morphology and size are than that in our previous experiment [10]. The the flow rate of the vapor phase and the CTiratio macrographs of whiskers clearly show a much The morphology and size of TiC whiskers are affec thicker TiC whisker layer to be growing on the ted by the flow rate of the vapor phase and the cti nickel substrate by the modified CVD method, ratio, Our experiment has confirmed that the effect while the morphologies of TiC whiskers prepared of the C/Ti on the whisker morphology is related to by the conventional method can only be observed the formation of Ni-Ti eutectic liquid phase with the aid of the microscope. The high yield and, cording to the VLS mechanism. Different whisker consequently, low cost of the whiskers prepared by morphologies are found at different locations of the this new method may be related to the movement of nickel substrate, this may be ascribed to the vari- the vapor phase in the vertical reaction tube, which ation of the contacting time and axial effective is different from that in the horizontal reactor. concentration of vapor phase along the vertical When the reactant vapor phases flow through reaction tube. The high yield of whiskers is closely a horizontal reaction tube, a laminar flow usually related to the movement of the vapor phase. A fast exists due to the density difference of different va- gas fluid spouting upwards accelerates the mass por components. This makes the radial reactant transition of the CVd process, and makes the concentration distribution inhomogeneous. It is radial concentration of the vapor phase more ho- difficult to obtain the methane and TiCl4 vapor for mogeneous. Furthermore, part of the vapor phases(5—500 lm), as shown in Fig. 6b. (3) Type C: Whiskers from location III have similar size as Type B, but they grew as a dot-distribution, as shown in Fig. 6c. The difference of whisker mor￾phology at different locations may be due to the difference of contacting time and axial effective vapor concentration along the nickel substrate cy￾linder. According to the gas movement with jet inlet gas [11—13], which is similar to the movement of vapor phase in the present experiment, from the bottom of the annulus in the vertical reaction tube to its top, the velocity of the vapor phase increases, and the residence time of the vapor phase decreases. Therefore, the effective vapor concentration and the time of flowing vapor phase contacting the growing whisker fronts decrease from the inlet to the top. In location I, the effective vapor concentra￾tion and the contacting time is the biggest, so the whiskers grow thick and long. In location II, the vapor concentration of the reactant is enough to form Ni—Ti binary eutectic liquid droplets as nu￾cleation sites of TiC whiskers, while the flowing vapor phase cannot completely contact the whisker tips. Thus in this location, the whiskers are thin and short (Fig. 6b). In location III, the contacting time and the effective concentration of vapor phase are the least, the whiskers grow as a thin, short, dot￾distribution due to incomplete growth and few nu￾cleation sites. From Table 1, it shows that the whisker yield in the modified CVD method increases several times than that in our previous experiment [10]. The macrographs of whiskers clearly show a much thicker TiC whisker layer to be growing on the nickel substrate by the modified CVD method, while the morphologies of TiC whiskers prepared by the conventional method can only be observed with the aid of the microscope. The high yield and, consequently, low cost of the whiskers prepared by this new method may be related to the movement of the vapor phase in the vertical reaction tube, which is different from that in the horizontal reactor. When the reactant vapor phases flow through a horizontal reaction tube, a laminar flow usually exists due to the density difference of different va￾por components. This makes the radial reactant concentration distribution inhomogeneous. It is difficult to obtain the methane and TiCl4 vapor for the whisker growth on the upper side of the nickel substrate cylinder. However, in a vertical reactor along with the greatly reduced gas inlet, a fluid jet blows vertically upwards, a spout form results to produce a fast fluid along the annulus where the formation reaction of TiC whiskers occurs, the re￾action gas velocity in the vertical tube increases. This not only eliminates the radial laminar, and makes the radial concentration distribution of va￾por phase more homogeneous, but also accelerates the CVD process of mass transition limit. On the other hand, apart from the fast gas blowing verti￾cally upwards, part of the vapor phases spouting from the gas inlet diverges into the annulus. Com￾pared to the vapors directly flowing through the substrate in the horizontal reaction tube, a higher percentage of the vapor phase is taken to form the deposits in the vertical reaction tube due to the divergence of the vapor jet. This increases the col￾lision of the vapor species on to the growing fronts and accelerates the mass transition of the CVD process. Therefore, the yield of the TiC whiskers prepared by the modified CVD method increases significantly. 4. Conclusions Quality TiC whiskers with a high yield are pre￾pared by a modified CVD method. The major fac￾tors affecting the whisker morphology and size are the flow rate of the vapor phase and the C/Ti ratio. The morphology and size of TiC whiskers are affec￾ted by the flow rate of the vapor phase and the C/Ti ratio, Our experiment has confirmed that the effect of the C/Ti on the whisker morphology is related to the formation of Ni—Ti eutectic liquid phase ac￾cording to the VLS mechanism. Different whisker morphologies are found at different locations of the nickel substrate, this may be ascribed to the vari￾ation of the contacting time and axial effective concentration of vapor phase along the vertical reaction tube. The high yield of whiskers is closely related to the movement of the vapor phase. A fast gas fluid spouting upwards accelerates the mass transition of the CVD process, and makes the radial concentration of the vapor phase more ho￾mogeneous. Furthermore, part of the vapor phases 590 Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591